FR2787015A1 - INTERVERTEBRAL DISC PROSTHESIS WITH COMPRESSIBLE BODY - Google Patents

Abstract

The invention concerns an intervertebral disc prosthesis comprising two plates (4) and a compressible body (12) interposed between the plates and having at least one end (18) with a contact zone with one of the plates. The body (12) and the plate (4) are so arranged that the contact zone has a surface area which increases when the stress of the plate towards the body is increased.

Description

The invention relates to intervertebral disc prostheses. We know

  according to document EP-0 356 112 an intervertebral disc prosthesis comprising two generally flat plates and an elastomer body interposed between the plates and fixed to these by its plane end faces. The mechanical behavior of this prosthesis is quite close to that of a healthy natural intervertebral disc, especially when the body

  is compressed between the two plates.

  An object of the invention is to provide a disc prosthesis of a different type and making it possible to approach the mechanical properties of a disc more closely.

healthy natural intervertebral.

  In order to achieve this goal, an intervertebral disc prosthesis is provided according to the invention comprising two plates and a compressible body interposed between the plates and having at least one end having a contact zone with one of the plates, in which the body and the plate are arranged so that the contact area has an area which increases with increasing stress on the

plateau towards the body.

  Thus, when the stress on the body in compression increases from a moderate or zero value, the height of the body, measured from one to the other

  of the trays, is reduced relatively quickly.

  Then, as the contact area increases, the body's reaction becomes more important and a comparatively greater stress must be exerted to reduce the height by an equivalent value. In other words, for the lowest compression values, the mechanical reaction of the body during compression varies very little as a function of the change in height. Consequently, the curve representing the stress exerted as a function of the variation in height is slightly inclined with respect to the horizontal for low compression values. Little effort is thus provided at the start of the race. This

  property is also that of a healthy natural disc.

  We can therefore adapt the behavior of the body not only by the choice of material but also by the shape of the end face or faces with contact area

  variable to get as close as possible to the mechanical properties of a healthy natural intervertebral disc.

  Advantageously, the contact zone is defined by one face of the plate and one face of the end of the body, one of the faces of the plate and of the body, in particular the face of the body, being curved and convex, and the other

face being flat.

  Advantageously, the contact zone is defined by one face of the plate and one face of the end of the body, the two faces being curved in at least one common direction and being respectively concave and convex, the concave face having at least a radius of curvature greater than a corresponding radius of curvature of the

convex face.

  Thus, this configuration makes it possible to implement the mechanical reaction variations as mentioned above. In addition, when the body is free to move laterally with respect to the plate, as will be seen below, this configuration ensures the relative centering of the two faces. For example, after the two faces have been offset, these curvatures

  allow them to automatically refocus.

  Advantageously, the body has at least one end in contact with one of the plates and free to move relative to the plate in a direction.

parallel to the plate.

  Thus, this arrangement reduces the risk of seeing too much stress appear between the two vertebrae in a direction perpendicular to the longitudinal direction of the spine, that is to say in shear. Advantageously, the body has one end housed in a recess in one of the plates capable of

  form a side stop for this end.

  Thus, one can limit the lateral displacements of the

  body in relation to the plate, even prohibit them.

  Advantageously, the body comprises a material

  viscoelastic, especially silicone.

  Such a material makes it possible to give a hysteresis shape to the curve representing the stress of the body in compression relative to its variation in height. This curve is also that of a healthy natural disc, so we approach its

mechanical properties.

  Advantageously, the prosthesis comprises a fluid

interposed between the plates.

  Thus, the addition of a fluid increases the hysteresis form, in particular when the fluid is compressible such as a gas or a mixture of a liquid and a gas.

  partially soluble in liquid.

  Advantageously, the fluid is in contact with the plates. Advantageously, the fluid extends around the periphery of the

body.

  Advantageously, the prosthesis comprises an enclosure containing the fluid and arranged so that it has a cross-sectional area parallel to the substantially invariable plates when varies a

  loading of the trays towards each other.

  Advantageously, the prosthesis is intended for the area

lumbar spine.

  Other features and benefits of

  the invention will also appear in the description

  following of a preferred embodiment and of two variants given by way of nonlimiting examples. In the accompanying drawings: - Figure 1 is a perspective view of a prosthesis according to the invention; - Figure 2 is an axial sectional view along the plane II-II of the prosthesis of Figure 1; - Figure 3 is an enlarged view of a detail D of Figure 2; - Figure 4 is a curve indicating the compressive force F exerted by the two plates on the cushion as a function of the variation of the distance separating them; - Figure 5 is a sectional view of a detail of an alternative embodiment of the prosthesis; and - Figure 6 is a simplified view similar to the

  Figure 2 showing a second alternative embodiment.

  The intervertebral disc prosthesis 2 according to the invention is here particularly intended for the lumbar region of the spine of the human body. It comprises two flat plates 4 having a general shape in the form of a rear hilum bean in plan view. Each plate 4 comprises a central circular plate 6 and a crown 8 extending around the periphery of the plate in the plane of the latter. At rest, the two plates 4 extend parallel to each other, at a distance and facing each other with their contours in coincidence. On each plate 4, the crown 8 and the plate 6 each have a groove 27 for the

receipt of a seal 31.

  The disc prosthesis 2 comprises a cushion or intermediate portion 10 interposed between the two plates 4. The cushion comprises a compressible solid body 12, here made of viscoelastic material, for example silicone. This body has a shore-A hardness advantageously between 60 and 100, and here about 80. The body 12 has a shape of revolution around its main axis 14. It has a cylindrical lateral face 16 and two end faces axial 18 generally perpendicular to axis 14 and of slightly spherical convex shape. Each face 18 therefore has two identical curvatures in planes perpendicular to each other. The body 12 is arranged coaxially with the plates 6. Each plate 6 has a flat internal central face 20 perpendicular to the axis 14 and in contact with one of the respective axial ends 18 of the body 12. Thus, the convex spherical face 18 of the body is supported on the flat face 20 of the plate. The body 12 is supported without anchoring on each of the plates 4 so that it is movable relative to each of these plates in a direction parallel to the plates, that is to say perpendicular to the main axis 14. We thus avoids the transmission of lateral stresses from one to the other

vertebrae.

  The cushion 10 further comprises a bellows 22. The bellows surrounds the body 14 coaxially therewith and at a distance therefrom. It has a symmetrical shape of revolution around the axis 14. Its wall has undulations in profile 24 making it possible to vary the length of the bellows 22 in the axial direction 14, without significantly varying the area of its section transversely to the axis 14. In this case, this bellows, like the plates 4, is made of titanium or titanium alloy, so that it has a certain axial rigidity and forms a compression spring. It can also be deformed in a direction perpendicular to the axis 14 or undergo a twist around the axis 14 or any axis perpendicular thereto. The bellows 22 has at its two axial ends edges glued to respective edges of the

  plates 6 projecting from the internal face 20.

  The bonding is carried out in a sealed manner so that the bellows 22 defines with the two plates 6 an enclosure

  waterproof with variable volume extending around the body 12.

  This enclosure contains a fluid, in this case a gas which is here air. The corrugations 24 closest to the body 12 extend away from the latter to allow free circulation of the gas from one to

the other of the plates 6.

  The bellows 22 has in this case ten convolutions, that is to say eight external ridges in addition to the two fixing ridges to the plates. Here it has an external diameter of approximately 30 mm and an internal diameter of approximately 17 mm. Its height, when the prosthesis is unloaded, is 10 mm. The wall of the bellows can be produced by means of one, two or three sheets each of 0.1 mm thick and the sum of the thicknesses of which forms the thickness of the wall. The bellows here has its own

stiffness of about 1.6 N / mm.

  Each ring 8 has two tabs 25 projecting from an external face of the plate 4 perpendicular to the plane of the plate. Each tab 25 has an orifice 27 passing right through it in the direction of the center of the plate and, on one face of the tab 25 opposite the plate 4, an impression of spherical shape. The holes 27 allow the reception of a bone screw 26 having a head 28 of which a lower face has a male spherical shape cooperating with the female imprint of the tab 25 to allow free orientation of the screw 26 relative to the tab associated. To achieve short-term anchoring of the disc prosthesis 2 in the column, the screws 26 can be anchored in the spondyle of the vertebrae adjacent to the

disc to be replaced.

  However, a so-called long-term anchoring may be provided, in addition, the surfaces of the plates 4 in contact with the adjacent vertebrae are covered with hydroxyapatite, or any other substance known per se which can stimulate bone growth. Before covering, said surfaces can be treated to obtain a more or less porous surface state, having anchoring points for the bone tissue, to ensure a better interface with said bone tissue. FIG. 4 shows the shape of the curve C indicating the intensity of a compressive force F exerted on the cushion 10 (that is to say on the two plates 4) without taking account of their deformability, almost zero, in the axial direction 14, depending on the variation of the length 1 of the cushion in the axial direction 14 (or even the distance between the two plates). This curve also represents the mechanical reaction R of the cushion under the same conditions. This curve C is not linear. In addition, it has a hysteresis form: the curve Ca indicating the increase in compression F from the zero origin being distinct from that Cd indicating the decrease in compression F until the origin, and s' extending entirely over the latter. This pronounced hysteresis form is mainly due to the viscoelastic material of the body and, in the alternative, to the association in the cushion.

of the body 12 and of the fluid.

  In addition, the curve Ca, relating to the increase in the compressive force F, presents from the origin O a portion Cal with a slight slope, then a portion Ca2 with a steeper slope. The curve Cd illustrating the reduction in compression F presents for the highest values of the force F a portion Cdl of steep slope, then for the lowest values of the force F a portion Cd2 of lower slope. The presence of a portion of slight slope in the vicinity of the origin for the curves Ca and Cd is mainly due to the conformation of the contact faces 18, 20 of the body 12 and of the plates 4, which results in the area of the area of mutual contact between each plate and the body, generally in the form of a disc, increases when the force F is increased. This increase occurs until reaching the maximum surface area of the contact zone, when the entire face 18

touches the board 4.

  The connection points Ja and Jd respectively form the junction between the curves Cal and Ca2, and Cdl and Cd2. On the curve Ca, the point Ja corresponds to the force F for which the maximum surfaces of

  contact between the plates and the body are reached.

  Similarly, on the curve Cd, the point Jd corresponds to the effort for which these surfaces cease to be maximum. The prosthesis can be configured so that the point Ja corresponds to a value of Al located between 25% and 75% of the maximum variation in length envisaged

for the prosthesis in use.

  With reference to FIG. 5, it may be provided in an alternative embodiment (also having the other characteristics of the prosthesis of FIG. 1) that the face 20 of each plate 4 facing the body 12 has a recess 32, here in "U", forming a lateral stop, in which is housed the corresponding axial end 18 of the body. This limits the relative lateral movements of the body 12 relative to each plate 4 to a certain range, or even

totally prohibited.

  In the variant of FIG. 6, the face 20 of the plate can be curved and concave in one or two directions, as is the case here, and the face 18 can be curved and convex in the corresponding direction or directions, the radius of curvature of the face 20 being, for each direction, greater than that of the face 18 in the corresponding direction. The two faces 18, 20 are here spherical. The radii of curvature of the surfaces 18 and 20 will for example be between 70 and mm, and 140 and 200 mm respectively. Such an arrangement makes it possible to obtain self-centering of the two faces while allowing relative lateral movement of the body 12 relative to the plate in any direction perpendicular to a longitudinal direction.

spine.

  In the embodiment of FIG. 2, the two ends of the body 12 have a contact surface 18 with the associated plate of variable surface area and making it movable laterally with respect to

to the body.

  On the contrary, in the variant of Figure 6, only one of the ends 18 of the body 12 has this property. The other end, lower in FIG. 6, has a circular planar shape with an invariable contact zone with the associated plate and fixed relative thereto. Of course, many modifications can be made to the invention without departing from the scope thereof. The fluid may be a liquid, or even a mixture of a liquid and a gas, the latter being for example

slightly soluble in liquid.

  The body may have an elliptical shape in

cross section to axis 14.

  The internal face 20 of the plates 4 may be convex, the axial end face 18 of the body 12 being flat, or concave with a radius of curvature greater than that of the face 20 of the plate. The two faces in

  contact of the plate and the body may be convex.

  The curvature of the faces can be limited to a single plane. We can implement the characteristics relating to the casing 22 (spring, distance to the body

  12) regardless of other characteristics.

-I he

Claims (11)

RESELL I CATIONS   1. Intervertebral disc prosthesis comprising two plates (4) and a compressible body (12) interposed between the plates and having at least one end (18) having a contact zone with one of the plates, characterized in that the body (12) and the plate (4) are arranged so that the contact area has an area which increases with increasing   loading of the platform towards the body.   2. Prosthesis according to claim 1, characterized in that the contact zone is defined by a face (20) of the plate and a face (18) of the end of the body, one of the faces of the plate and of the body , in particular the face (18) of the body, being curved and   convex, and the other face (20) being planar.   3. Prosthesis according to claim 1, characterized in that the contact zone is defined by a face (20) of the plate and a face (18) of the end of the body, the two faces being curved in at least one common direction and being respectively concave and convex, the concave face (20) having at least one radius of curvature greater than a corresponding radius of curvature of the convex face (18).   4. Prosthesis according to any one of   claims 1 to 3, characterized in that the body   (12) has at least one end (18) in contact with one of the plates (4) and free to move relative to the plate in a direction parallel to the tray.   5. Prosthesis according to any one of   claims 1 to 4, characterized in that the body   (12) has one end (18) housed in a recess (32) of one of the plates (4) capable of forming   a lateral stop for this end.   6. Prosthesis according to any one of   claims 1 to 5, characterized in that the body   (12) comprises a viscoelastic material, in particular silicone. 7. Prosthesis according to any one of   claims 1 to 6, characterized in that it   includes a fluid interposed between the plates.   8. Prosthesis according to claim 7, characterized   in that the fluid is in contact with the plates.   9. Prosthesis according to claim 7 or 8, characterized in that the fluid extends around the periphery of the body (12).   10. Prosthesis according to any one of   claims 7 to 9, characterized in that it   comprises an enclosure (22) containing the fluid and arranged so that it has a cross-sectional area parallel to the plates (4) substantially invariable when a stress on the plates varies towards each other.   11. Prosthesis according to any one of   Claims 1 to 10, characterized in that they are   a lumbar intervertebral disc prosthesis.